This invention generally relates to video signal processing and is specifically directed to enhancement of an analog video signal produced for display from information stored in digital form.
For various applications, such as in a slow-scan television system, it is desirable to perform a scan conversion process on pictorial information such that the rate of scanning the information is changed from a typical broadcast television scan rate to a slower rate for transmission of the information and then back again to the original rate for display on a television monitor. To accomplish this scan rate conversion, a memory for storing the picture information that is scanned or transmitted at the slower rate is required so that an entire picture may be viewed at one time. This is because the picture information is scanned and/or transmitted at too slow a rate for the entire picture to be displayed immediately upon receipt of the transmission within the persistence time of phosphors used on the face of television tubes. A digital memory is preferred because of its precise retention and accurate reproduction. A slow-scan television apparatus including a digital memory is described in an article by Steber, "SSTV to Fast Scan Converter", Parts I and II, QST, March 1975, pp. 33-40 and May 1975, pp.28-46.
In digital storage, the picture information is represented in discrete memory cells. This means that the picture is "quantized" both in space (two dimensions, x and y), and in grey shades. Because it is desirable to minimize the cost of the memory, and to minimize information transmission time, it is desirable to use as few memory cells as possible to represent the picture. While the possible arrangements can vary widely, in the following discussion, an example is considered, wherein the information stored in the memory represents a picture composed of 128 horizontal scan lines, each of which contains 128 picture element locations and 16 grey shades for each picture element intensity value. Since it takes 4 bits to represent 16 grey shades, the required memory capacity is 128 times 128 times 4 memory cells.
An analog video signal produced from information stored in such a memory for display on a television monitor would include 128 horizontal scan lines, with each line consisting of 128 picture element locations, as shown in FIG. 1. The picture array is square (128 times 128). However, because information typically is filled in along scan lines between the picture element locations by lateral extension of each picture element location, but not filled in between the scan lines, the scan lines are too noticeable. Therefore, it is desirable to "fill in" information between the scan lines. This can be done by simply displaying each horizontal scan line twice in succession, as shown in FIG. 2. In FIG. 2, the scan lines labeled 1a, 2a, etc. provide exactly the same information as do the previously displayed lines (1, 2, etc.), thus reducing the gap between the displayed scan lines.
However, the visual quantization effect produced by this kind of display is that of tiny "squares" in the picture, as indicated by box 10 in FIG. 2. An example of a portion of such a picture is illustrated in FIG. 3.
Attempts have been made to reduce this "quantization" effect in slowscan television apparatus. For example, Steber describes a system for "smoothing" the picture by providing that the intensity in each picture element location of the intermediate scan lines (1a, 2a, etc.) represents the averageof the intensities in the adjacent picture element locations that are in the scan lines above and below (1, 2, etc.). Additionally, or alternatively, it has been suggested that the picture element locations can be broken in half horizontally and that the intensity of one of the halves be made to be the average of the intensities of the two half-elements on either side thereof. For example, the intensity in element location 1c would be the average of the intensity in element locations 1b and 2b.
This process is roughly equivalent to low-pass filtering the picture and the resultant effect is to "soften" the picture and make it appear less "sharp" . Such an effect is not necessarily desirable.